In the Majorana equation for particles with arbitrary spin, the wave packet is due not only to the uncertainty affecting position and momentum but also to the infinite components with decreasing mass forming the Majorana spinor. In this paper we prove that such components contribute to increase the spreading of the wave packet. Moreover, as occurs in the time propagation of the Dirac wave packet, also in that of Majorana the Zitterbewegung takes place, but it shows a peculiar fine structure. Finally, the group velocity always remains subluminal and the contributions due to the infinite components decrease progressively increasing the spin.

In this study the tachyon-like Dirac equation, formulated by Chodos to describe superluminal neutrino, is solved. The analytical solutions are Gaussian wave packets obtained using the envelope method. It is shown that the superluminal neutrino behaves like a pseudo-tachyon, namely a particle with subluminal velocity and pure imaginary mass that fulfils the energy-momentum relation typical of classical tachyons. The obtained results are used to prove that the trembling motion of the particle position around the median, known as Zitterbewegung, also takes place for the superluminal neutrino, even if the oscillation velocity is always lower than the speed of light. Finally, the pseudo-tachyon wave packet is used to calculate the probability of oscillation between mass states, obtaining a formula analogous to the one obtained for the ordinary neutrino. This suggest that in the experiments concerning neutrino oscillation is not possible to distinguish tachyonic neutrinos from ordinary ones.